The present invention relates to a process for preparing biaryls using catalysts based on palladium compounds with phosphite ligands.
Biaryl compounds, in particular biphenyl compounds, are industrially important as fine chemicals, intermediates for pharmaceuticals, optical brighteners and agro-chemicals.
A method which is frequently employed for the synthesis of biaryls on a laboratory scale is the Suzuki reaction in which iodoaromatics or bromoaromatics or in exceptional cases chloroaromatics are reacted with arylboronic, vinylboronic or alkylboronic acid derivatives in the presence of palladium catalysts. Review articles describing this methodology may be found, for example, in N. Miyaura, A. Suzuki, Chem. Rev. 1995, 95, 2457. Catalysts used for the purposes of the Suzuki reaction are in general palladium and nickel compounds. Despite the economic advantage of nickel catalysts (cf. A. F. Indolese, Tetrahedron Lett. 1997, 38, 3513), palladium catalysts are preferred to nickel catalysts because of the lower toxicity and the greater tolerance to functional groups. When using palladium catalysts, both palladium(II) and palladium(0) complexes are employed in Suzuki reactions (cf. M. Beller, H. Fischer, W. A. Herrmann, K. xc3x96fele, C. Broxcex2mer, Angew. Chem. 1995, 107, 1992). According to the literature, coordinatively unsaturated 14- and 16-electron palladium(0) species stabilized by means of donor ligands such as phosphines are formulated as catalytically active species. Particularly when using relatively low-cost starting materials such as aryl bromides or aryl chlorides, it is necessary to add stabilizing ligands in order to achieve a satisfactory catalytic activation of the starting materials.
A substantial disadvantage of the Suzuki reactions described is that satisfactory catalytic turnover numbers (TONs) can be achieved only when using uneconomical starting materials such as iodoaromatics and activated (i.e. electron-deficient) bromoaromatics. Otherwise, when using deactivated (i.e. electron-rich) bromoaromatics or chloroaromatics, large amounts of catalysts, usually from 1 to 5 mol %, have to be added so as to achieve industrially usable conversions. Owing to the complexity of the reaction mixtures, simple catalyst recycling is also not possible, so that catalyst costs, too, generally stand in the way of industrial implementation. Relatively recent catalyst systems based on water-soluble phosphines do give satisfactory catalytic activities in the industrially important reaction of 2-chlorobenzonitrile with p-tolylboronic acid, but the catalysts comprise relatively expensive sulfonated phosphines. Furthermore, a number of chloroaromatics cannot be activated in an industrially satisfactory manner even by means of these catalysts (cf. S. Haber, Fine Chemical Syntheses, in B. Cornils, W. A. Herrmann, Aqueous Phase Organometallic Catalysis, Wiley-VCH: Weinheim, N.Y., Chichester 1998, p. 440 ff.).
It is an object of the present invention to provide a novel process for preparing biaryls which does not display the disadvantages of the known processes, is suitable for industrial implementation and gives biaryls in high yield, catalyst productivity and purity.
This object is achieved by a process for preparing monofunctional, bifunctional and/or polyfunctional biaryls of the formula (I)
Arxe2x80x94Arxe2x80x2xe2x80x83xe2x80x83(I)
where Ar and Arxe2x80x2 are each, independently of one another,
an aromatic radical having up to 14 carbon atoms or
a heteroaromatic selected from the group consisting of five-, six- or seven-membered rings having at least one nitrogen, oxygen and/or sulfur atom in the ring;
by reacting haloaromatics of the formula (II)
Arxe2x80x94Xxe2x80x83xe2x80x83(II)
with boron compounds of the formula (IIIa), (IIIb) and/or (IIIc) 
where, in the formulae (II), (IIIa), (IIIb) and (IIIc),
Ar and Arxe2x80x2 are as defined for formula (I);
X is selected from the group consisting of chlorine, bromine, iodine, OSO2CF3, OSO2aryl-(C6-C10), OSO2alkyl-(C1-C8) and N2+Yxe2x88x92, where Y is a chlorine, bromine or iodine atom or a tetrafluoro-borate or tetraphenylborate anion;
Q1 and Q2 are selected independently from the group consisting of OH, fluorine, chlorine, bromine, iodine, alkyl-(C1-C4), aryl-(C6-C10), alkoxy-(C1-C4) and aryloxy-(C6-C10);
in the presence of at least one palladium complex of the formula (IVa) or (IVb), 
where
the radicals R1 to R4 are each, independently of one another, a (C1-C18)-alkyl radical or one of the above-described radicals Ar;
E is a carbon bridge having from two to seven carbon atoms; and
n is an integer from 1 to 4.
In a further embodiment of the invention,
the aromatic radicals Ar and Arxe2x80x2 have up to eight substituents;
the heteroaromatic has up to five substituents; and/or
the radicals R1 to R4 have up to eight substituents which are selected independently from the group consisting of fluorine, chlorine, CF3, OH, NO2, CN, R5, Oxe2x80x94R5, CHO, COxe2x80x94R5, COOH, COOxe2x80x94R5, OCOxe2x80x94R5, SiR53, NH2, NHxe2x80x94R5, Nxe2x80x94R52, SOxe2x80x94R5, SO2xe2x80x94R5, SO3H, SO3xe2x80x94R5, CONH2, NHCOH, NHCOxe2x80x94R5, NHCOOxe2x80x94R5, CHCHxe2x80x94CO2-alkyl-(C1-C8), POxe2x80x94R52, Pxe2x80x94R52, PO3H2, PO(O-alkyl-(C1-C6))2 and CHCHCO2H; where R5 is an alkyl radical having from 1 to 8 carbon atoms or an aryl radical having from 6 to 10 carbon atoms, e.g. phenyl.
In the formula (IVb), the carbon bridge E can have up to seven substituents selected independently from the group consisting of (C1-C4)-alkyl, O-alkyl-(C1-C4), OH and Ar, where Ar is as defined for formula (I).
It is likewise possible for Q1 and Q2 in the formula (IIIa) each to be, independently of one another, a (C1-C4)-alkyl, (C1-C4)-alkoxy, (C6-C10)-aryl or (C6-C10) -aryloxy radical which is substituted by at least one halogen atom or a (C1-C4)-alkoxy or (C1-C4)-alkyl radical; or for Q1 and Q2 in the formula (IIIa) together to form an alkylenedioxy or alkylene group which has from one to four carbon atoms and may be substituted by up to four (C1-C4)-alkyl and/or (C6-C10)-aryl radicals.
The radicals Ar and Arxe2x80x2 can each be, independently of one another, a heteroaromatic in which up to four further aromatic, heteroaromatic and/or aliphatic rings are fused onto the heteroaromatic ring. The process of the invention is particularly suitable for the synthesis of biaryls in which Ar and Arxe2x80x2 are each a substituted phenyl, naphthyl, anthryl, phenanthryl, biphenyl radical and/or a five-, six- or seven-membered heteroaromatic having nitrogen, oxygen or sulfur atoms in the ring. In the case of hetero-aromatics, particular preference is given to hetero-aromatics such as substituted pyridines, pyrimidines, oxazoles, imidazoles, pyrazines, quinolines, indoles, furans, benzofurans and/or thiophenes.
The process of the invention has been found to be particularly useful for preparing compounds of the formula (I) in which the radicals Ar and Arxe2x80x2 each have, independently of one another, up to 5 substituents selected from the group consisting of alkyl-(C1-C8), O-alkyl-(C1-C8), OCO-alkyl-(C1-C8), N-alkyl2-(C1-C8), phenyl, aryl, fluorine, chlorine, NO2, CN, COOH, CHO, SO2-alkyl-(C1-C4), NH-alkyl-(C1-C8), COO-alkyl-(C1-C8), CONH2, CONH-alkyl-(C1-C8), CO-alkyl-(C1-C8), CO-phenyl and PO-phenyl2.
The reaction generally takes place in the presence of at least one solvent selected from the group consisting of water, aliphatic ethers, aromatic or aliphatic hydrocarbons, alcohols, esters, aromatic or aliphatic nitrites and dipolar aprotic solvents such as dialkyl sulfoxides, N,N-dialkylamides of aliphatic carboxylic acids or alkylated lactams. The solvent is preferably THF, dioxane, diethyl ether, diglyme, MTBE, DME, acetonitrile, toluene, xylenes, anisole, ethyl acetate, methanol, ethanol, butanol, ethylene glycol, ethylene carbonate, propylene carbonate, dimethyl sulfoxide, N,N-dimethylacetamide, N,N-dimethylformamide or N-methylpyrrolidone.
Since an acid is formed in the reaction, it is advantageous to neutralize the resulting acid by addition of a base. Bases suitable for this purpose are:
primary, secondary and tertiary amines such as alkylamines, dialkylamines, trialkylamines, each of which may be alicyclic or open-chain;
alkali metal and alkaline earth metal salts of aliphatic or/and aromatic carboxylic acids, e.g. acetates, propionates, benzoates, in particular their carbonates, hydrogen-carbonates, phosphates, hydrogenphosphates or/and hydroxides; and
metal alkoxides, in particular alkali metal or alkaline earth metal alkoxides, e.g. sodium methoxide, potassium methoxide, sodium ethoxide, magnesium methoxide and calcium ethoxide.
The base is most preferably a carbonate, hydroxide or phosphate of lithium, sodium, potassium, calcium, magnesium or cesium.
Apart from neutralization of the acid formed, the base used can also have a positive influence on the course of the reaction by activating the arylboronic acid to form anionic boranate species. Apart from use of the abovementioned bases, such activation can also be achieved by addition of fluoride salts such as CaF2, NaF, KF, LiF, CsF or (C1-C8)-alkyl4NF.
The palladium catalysts of the formula (IVa) or (IVb) which are used are generally produced in situ from at least one palladium(II) salt or a palladium(0) compound and the corresponding phosphite ligands. However, they can also be used directly as palladium(0) compound without the initial catalytic activity being reduced thereby. The palladium(II) salt is preferably selected from the group consisting of palladium(II) acetate, palladium(II) chloride, palladium(II) bromide, lithium tetrachloropalladate(II), palladium(II) acetylacetonate, palladium(II) propionate, bis(acetonitrile)palladium(II) chloride, bis(triphenylphosphine)palladium(II) dichloride and bis(benzonitrile)palladium(II) chloride. The palladium(0) compound is, in particular, selected from the group consisting of palladium(0) dibenzylideneacetone complexes, tetrakis(triphenyl phosphite)palladium(0), tetrakis(triethyl phosphite)-palladium(0), tetrakis(triphenylphosphine)palladium(0), bis(tri-o-tolylphosphine)palladium(0), tricyclohexyl-phosphinepalladium(0) diallyl ether complex and bis(tricyclohexylphosphine)palladium(0).
The molar ratio of palladium to phosphite ligand should be less than 1 and is preferably in the range from 1:2 to 1:1 000, in particular from 1:5 to 1:200.
When using chloroaromatics, bromoaromatics, aryl triflates and/or aryl mesylates or related starting materials, it may be advantageous to use a cocatalyst in addition to the palladium-phosphite catalyst. This cocatalyst is selected from the group consisting of halogen salts, in particular a halide of the alkali metals and/or alkaline earth metals, an ammonium halide and/or a phosphonium halide, preferably a bromide or/and chloride. Particularly preferred halogen salts are LiBr, LiCl, NaBr, KBr, CsBr, Bu4NCl, Bu4NBr, benzyltrimethylammonium bromide, trioctylmethylammonium bromide or tetraphenylphosphonium bromide. The cocatalyst is usually used in an amount of from 0.001 mol % to 100 mol %, in particular from 0.01 to 50 mol %, based on the compound of the formula (II). If there are process engineering advantages, the reaction can also be carried out in the cocatalyst as solvent (salt melt).
The reaction is generally carried out at a temperature of from 20 to 200xc2x0 C., preferably from 60 to 180xc2x0 C., in particular from 80 to 160xc2x0 C., and at a pressure of up to 100 bar, preferably at a pressure in the range from atmospheric pressure to 60 bar.
The process of the invention enables turnover numbers for the catalysts of the order of 100 000 and more for bromoaromatics as starting materials and 10 000 and more for chloroaromatics to be achieved. These values correspond to the best known results which have been achieved using phosphine ligands.
Owing to the catalyst activities and stabilities, the process of the invention makes it possible for the first time to use extremely small amounts of catalyst so that the catalyst costs are not cost-limiting, in contrast to the known Suzuki reactions for the corresponding process.
In the process of the invention, catalyst contents of from 1 to 2 mol % are used in exceptional cases; they are usually  less than 1 mol %, particularly preferably  less than 0.2 mol %. Furthermore, phosphite ligands can be prepared more simply and inexpensively than the phosphine ligands used hitherto and they are easier to modify and more stable to oxidation reactions.
The biaryls prepared according to the invention are used, inter alia, in industry as intermediates for pharmaceuticals (ATII antagonists) and agrochemicals, as ligand precursors for metallocene catalysts, as optical brighteners and as building blocks for polymers.
The following examples illustrate the process of the invention without restricting it thereto.